Invective deals with one addition (7)
I believe the answer is:
tirades
'invective' is the definition.
(I've seen this before)
'deals with one addition' is the wordplay.
I cannot really understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'deals' could be 'trades' ('trade' can be a synonym of 'deal') and 'trades' is found within the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tirades that I've seen before include "Long angry speeches of criticism or accusation" , "Rants" , "Invective" , "Astride (anag) - diatribes" , "Denunciations" .)